This invention relates to a method and apparatus for providing an improved control of the steam temperature at the outlet of a boiler. More particularly, the invention provides for a method and apparatus for maintaining closer control of steam temperature at the outlet of a drum type boiler when that boiler is being operated in a sliding or variable pressure mode; that is, when the pressure is allowed to vary with changes in load on the boiler.
In boilers fired for the production of electrical power, the generation of steam takes place in two stages. Generation of saturated steam takes place in the evaporator section of the boiler and the saturated steam produced exists in the boiler drum along with water which is maintained at a desired level. The saturated steam is superheated to the desired temperature by one or more superheating sections in the boiler so that the steam generated at the outlet of the boiler is at appropriate pressure and temperature for use in a turbine connector to drive a generator.
Typically, the steam temperature at the boiler outlet must be controlled for proper operation of the turbine and the control of that temperature may be accomplished in various ways, depending upon the structural features of the boiler. Basically the control of steam temperature is accomplished by either modifying the fluid distribution in the boiler or by modifying the heat distribution. Thus, for example, the steam temperature may be controlled by modifying the amount of spray which is used to desuperheat or, alternatively, the steam or water may be bypassed around certain sections in the boiler as another means for modifying the temperature by changing the fluid distribution. Where it is desired to modify the heat distribution to control steam temperature, various approaches can be used; for example, tilting the burners or biasing the fuel supply to the upper burners with respect to the lower burners. Alternatively, gas recirculating can be controlled as a means for controlling the heat distribution or other means for gas flow distribution can be utilized.
It has been found that the pressure at which the boiler operates has a profound effect on the temperature simply by virtue of the characteristics of steam as represented by the heat of vaporization, the enthalpy of saturated steam at the drum, and the enthalpy in the superheated steam for a given temperature. It is the object of this invention to provide a method and means to compensate for the sustained effects of pressure variations on the temperature of the steam at the boiler outlet.